Electronic skin having uniform gray scale reflectivity

ABSTRACT

The invention features electronic skin including an active layer formed of bistable cholesteric liquid crystal material and articles comprising the skin, the electronic skin having uniform gray scale reflectivity. Also featured is method for producing suitable reduction pulses that will provide a display (e.g., the electronic skin) with uniform gray scale reflectivity. Reduction pulses of narrow width are used to create uniform levels of gray in the electronic skin to overcome display imperfections that cause discontinuity in the gray scale reflectivity.

This application is a divisional of U.S. patent application Ser. No.12/152,862, filed on May 16, 2008 now U.S. Pat. No. 8,310,630, which isincorporated herein by reference.

FIELD OF THE INVENTION

The invention pertains to gray scale of electro-optic devices and, inparticular to achieving uniform gray scale of liquid crystal devicesthat are susceptible to gray scale discontinuities, therefore enablingelectronic skins that can exhibit uniform tunable colors over a largearea.

BACKGROUND OF INVENTION

Bistable cholesteric liquid crystal displays were introduced in theearly 1990's (see U.S. Pat. Nos. 5,437,811 and 5,453,863). Theirzero-power image retention and sunlight readability led to theirintegration into numerous signs and battery-powered applications asreviewed in “Cholesteric Liquid Crystals for Flexible Displays” inFlexible Flat Panel Displays, Ed. G. Crawford, (John Wiley & Sons, 2005)J. W. Doane and A. Khan, Chapter 17. The technology is best suited forreflective color images. In the cholesteric display technology,multiple-color and full-color displays are preferably produced bystacking multiple cholesteric liquid crystal layers with each tuned toreflect a different wavelength, typically red, green, and blue (see U.S.Pat. No. 6,654,080). These three colors are additively mixed to achieveup to eight colors. Images with more colors are possible because thetechnology is amenable to grayscale. That is, the reflective brightnessof each color can be electronically adjusted to any desired levelbetween the display's maximum and minimum brightness. Each level ofbrightness is referred to as a gray level. The total number of colorsdepends upon the number of gray levels one can choose for each colorlayer. High resolution displays with as many as 4096 colors have beenproduced.

Commercial bistable cholesteric displays of the prior art displaydigital images and as such are made using of a matrix of pixels witheach of the pixels having a small area. The resolution of the displaydepends upon the number of pixels and size of the display. Typical pixelsizes are substantially less than one square millimeter. These displaysare typically manufactured on glass substrates. Recent progress has beenmade in commercializing displays built on flexible plastic substratesrather than glass. The new flexible displays are manufactured with asimple lamination process, and may be cut into interesting shapes afterassembly. Of significance, these displays are very thin since thinplastic sheet material as thin as 12.5 microns can be used for thesubstrates making possible a display with the over all thickness lessthat 60 microns. Using cholesteric liquid crystals dispersed asemulsified droplets has made possible even thinner displays since allthe materials of the display including the electrodes, substrates andcholesteric dispersion can be coated in thin layers.

Such developments suggest a display film that can be electronicallyswitched from one color to another color that can be laminated to flatsurfaces and even made to conform to curved surfaces in the form of askin. Consumers frequently identify color as a necessity for severaltypes of products, such as; clothing, accessories, hand held electronicssuch cell phones, personally worn electronics, medical indicators, anddecorative items. The color on these items is defined on the productwhen purchased. Conventionally, it has not been possible toelectronically change the color of these items after the initialpurchase. Thin flexible displays for changing the color of articles, forexample, an electrochromic layer or a cholesteric display skin forchanging the color of cell phones, have been described in the patentliterature but such devices have not been successfully implemented(Published Patent Application No. 2008/0074383 and U.S. Pat. No.7,142,190). Such cholesteric display skins would suffer from a problemof gray scale discontinuity discussed below. Other products incorporatea color change indicator for either a sensorial signal to indicate theproduct is properly working or to indicate the user's attention isrequired. Several color indicator products exist such as battery testers(U.S. Pat. No. 7,188,996) and self expiring security badges (U.S. Pat.No. 6,752,430).

Cholesteric display films have not been suitable for electronic skinapplications with tunable uniform colors because uniform gray levelshave not been possible in areas around one square centimeter and larger.In areas of such size, the inventors have noticed that levels of graybecome very non-uniform or blotchy in appearance. The reason for this isnot completely understood but it is believed by the inventors to be aresult of several possible causes such as: non-uniform cell gapthickness (varying distance between electrodes) and non-uniformconductivity of transparent electrodes. Such features have not been aproblem in typical cholesteric matrix displays because the pixels are sovery small that gray levels appear uniform on the scale of a pixel andimage content makes the non-uniformity hard to detect across manypixels. Furthermore, nearly all commercial displays have been driven ina binary (on/off) mode not utilizing shades of gray.

Cholesteric Liquid Crystal Display Prior Art:

Prior art bistable cholesteric display technology has been used ondigital displays incorporating a matrix of small pixels. The mostsuitable matrix for the cholesteric technology has been the passivematrix because of a unique voltage threshold in its electrooptic voltageresponse curve. A passive matrix is a plurality of row electrodes on onesubstrate and a plurality of column electrodes on the opposing substrateorthogonal to the row electrodes. Intersection of the row and columnelectrodes forms a matrix of pixels of the liquid crystal materialbetween the electrodes. As the electrode rows and columns are very thinin such passive matrix displays, each of the pixels has a very smallarea, of the order of a square millimeter or much less, providing thedisplay with high resolution. Row and column voltage waveforms areapplied to the pixels using row and column driver circuitry, changingthe reflective states of the pixels. The voltage thresholdcharacteristic of a bistable cholesteric material allows the pixels ineach row to be independently addressed while unaffecting the others. Apassive matrix display is addressed a row at a time until the entiredisplay is addressed with an image. Because of bistability, the image isretained on the display indefinitely or until a new image is addressedon the matrix.

Referring to FIG. 1, a typical drive scheme for cholesteric displays ofthe prior art involves erasing the display to either a planar (bright)or focal conic (dark) texture and then driving the display to itsdesired brightness with a drive pulse. This figure shows the resultantreflectivity of driving a typical bistable cholesteric display cell ofthe prior art starting from the planar state of maximum reflectivity,curve 10, as well as from the focal conic state of minimum reflectivity,curve 11. The focal conic state is a forward scattering state that isessentially transparent to the light traveling through it. Each point incurve 10 was achieved by first applying an erase pulse (sometimesreferred to as a clearing pulse; see U.S. Pat. No. 5,644,330 as well aslater discussion in this document) to drive the display to its highestreflectivity in the planar texture as described in the prior art.Following each erase pulse a 100 ms drive pulse (frequency=250 Hz) isapplied to achieve the recorded reflectivity. Each point in curve 11 waslikewise achieved by first applying an erase pulse (sometimes referredto as a clearing pulse; see U.S. Pat. No. 5,644,330), however drivingthe display cell to its darkest (least reflective) focal conic state.Following each erase pulse a 100 ms drive pulse (frequency=250 Hz) isapplied to achieve the measured reflectivity recorded as the normalizedreflectance. The horizontal axis shows the root mean square (rms)voltage of the applied pulse.

In operation of a prior art cholesteric liquid crystal display havingthe electrooptic response curve shown in FIG. 1, a pulse with voltageslightly less than or equal to V3, indicated by 13, may be used toaddress the darkest pixels (i.e., to place pixels desired to be darkinto the focal conic state). A voltage>V4, indicated by 12 on curve 10,may be used for addressing bright pixels (i.e., to place pixels desiredto be bright into the planar state). Gray scale levels are those levelshaving a reflectivity between the planar and focal conic states. Grayscale may be achieved by using pulses with values between V3 and V4.Reflectivity of prior art cholesteric displays is highly sensitive tovoltage as seen by the extreme steepness of the curve in the gray scaleregion between V3 and V4 (FIG. 1). Thus, the prior art gray scale drivescheme is sensitive to applied voltage and display imperfections.

Prior art drive pulses may have many forms (see for example U.S. Pat.Nos. 6,268,839 and 5,933,203). Typical but not exhaustive drive pulseforms are dc, ac, and ac with a pause between phases, illustrated inFIGS. 2 and 3. A unipolar dc pulse 21 is illustrated in FIG. 2 a; an acpulse 22 in FIG. 2 b and an ac pulse 23 with a pause between in FIG. 2c. It is appreciated also that the ac examples may consist of eithersingle or multiple periods; that is, more than one pulse, in order tosatisfy drive frequency requirements of the liquid crystal materials. Asshown in FIGS. 3 a, 3 b and 3 c, in some circumstances pulse widthmodulation between two voltage levels (labeled V0 and V1) may be used toadjust the rms value of the drive pulses as illustration by pulses 31,32, and 33.

Any of these prior art drive pulses can be used as drive and reductionpulses in the present invention. The width of a unipolar pulse of FIG. 2a is the distance between the leading and trailing ends of the pulse;the width of bipolar pulse of FIG. 2 b is the distance between theleading end of the positive polarity pulse portion and the trailing endof the negative polarity pulse portion; the width of the bipolar pulseof FIG. 2 c is the same as in FIG. 2 b but includes the interveningpause at zero voltage between positive and negative polarity pulseportions; the widths of the pulse width modulated pulses of FIGS. 3 a, 3b and 3 c are the same as in FIGS. 2 a, 2 b and 2 c, respectively.

U.S. Pat. No. 6,133,895 discloses a cumulative drive scheme for acholesteric liquid crystal display for changing images on the display ata near video rate. This driving procedure takes advantage of thecumulative nature of a cholesteric display as well as its thresholdcharacteristic to address the pixels of a passive matrix with a sequenceof pulses of narrow width in the same manner as an regular liquidcrystal display (LCD) in the twisted nematic (TN) or super twistednematic (STN) mode to provide the same video image aesthetic. Because ofthe higher viscosity of cholesteric liquid crystals video rates are onlypossible however on small matrices where the number of pixels islimited. FIGS. 4A and 4B of U.S. Pat. No. 6,133,895 show increases inreflectivity and FIGS. 5A and 5B of the patent show decreases inreflectivity, using AC pulses at a constant voltage and 1 ms pulsewidth.

SUMMARY

The invention achieves uniform gray scale for areas of bistablecholesteric liquid crystal displays that are susceptible to many grayscale non-uniformities or discontinuities. The gray scalediscontinuities are found in areas of the display in which blotchinessin gray scale reflectivity is noticeable with the naked eye, forexample, for areas of the display bounded by conductive layers that areabout 1 cm² and larger and in particular, at least (2.54 cm)² or (1.00in²) (referred to herein as “display areas”). Typically a display areais the entire viewing area of the display, but it can be a portion of adisplay, such as in the case of a display having a portion withunpatterned conductors and another portion with patterned conductors. Inthis disclosure, uniform levels of gray in the display area areaccomplished by a method of electronically driving different shades ofgray using a sequence of voltage reduction pulses. A particular shade ofgray is determined by the number of such reduction pulses.

We further disclose an electronically tunable color overlay liquidcrystal display referred to in this disclosure as electronic skin thatcan be placed on anything. Electronic skin is a sheet of material,usually a flexible material, that can be formed into a sleeve or haveadhesive on the back to form a sticker of changeable color to be placedon any item the user desires a different color. This invention willallow consumers to individualize anything by adding a personalizedchangeable color to the item of interest. Several possible items toattach electronic skin to are described. The reflection of the frontsurface of the electronic skin can be shiny like plastic or can havevaried textures to the front surface reflection.

Electronic skin can encompass components of a bistable cholestericdisplay cell made of stacked layers to create a multitude of colors asdescribed for example in U.S. Pat. Nos. 6,654,080, 6,377,321 and7,061,559. These patents describe cholesteric displays that can create afull color image that remains on the display without applied power untilerased and addressed with a new image. The stacked liquid crystal layersof the electronic skin are in the form of a film or sheet. The entiresurface of the display can change from one color to another. The colorsof each layer in the electronic skin may be tuned to create a colorspace of particular interest such as; pink, violet, skin tones on colorsmatching garments. The color of the layers making up the entireelectronic skin may be red, green, and blue or different colors. Theelectronic skin may be include a triple, double or even single liquidcrystal layer if fewer colors are desired (e.g., a single active layerwith a color back layer). The electronic skin can be made of flexiblesubstrates such as plastic so that the skin can conform to the surfaceof the object on which it is laminated. Flexible cholesteric displaysare briefly described in the book “Cholesteric Liquid Crystals forFlexible Displays” in Flexible Flat Panel Displays, Ed. G. Crawford,(John Wiley & Sons, 2005) J. W. Doane and A. Khan, Chapter 17. Theelectronic skin is a form of display but may only provide a colorwithout necessarily an image. The electronic skin may be thermoformed toconform to the desired shape and may possess unpatterned electrodes.

In this disclosure, a new drive scheme is described for cholestericliquid crystal displays. The drive scheme is applicable to glass- orplastic-based displays. Driving the display with the new schememinimizes the sensitivity of the gray levels to display imperfectionsand reduces the need for further improvements in display manufacturingprocesses.

The electronic skin avoids gray scale discontinuities caused byimperfections in the display that may arise by: imperfections in theelectrodes; non-uniform cell gap; non-uniform polymer dispersions; nonuniform structure of the cholesteric liquid crystal such as caused bynon-uniform surface induced alignment or other means. Reduction voltagepulses of narrow width are used to reduce the sensitivity of gray levelsso that imperfections in the display are not visible in the gray levelstherefore providing a uniform reflective intensity of a Bragg reflectivecolor. In prior art displays having a liquid crystal sandwiched betweentwo substrates, cell gap is the measured distance between thesubstrates. Such prior art displays normally have electrodes patternedon the inside of each substrate in contact with the liquid crystallayer, the electrodes being extremely thin on the order of angstroms.Prior art substrates of liquid crystal displays are normally made ofglass and are very thick in comparison to the very thin liquid crystallayer, which can be on the order of 10 microns or less in thickness.Cell gap is used herein to describe a close approximation of thedistance between electrodes. Imperfections in the cell gap spacing arecommon in the manufacturing of cholesteric displays, and can be created,for example, in the following situations: by substrates that aren'tsuitably smooth, by substrates that are soft and easily distorted in themanufacturing process or later on (e.g., flexible plastic substrates),by dust particles between the substrates, by application of non-uniformpressure in the sealing of the cells (e.g., compression of glasssubstrates during roller leveling of the liquid crystal material filledbetween the substrates), or during curing of droplet dispersions of theliquid crystal material filled in the cell. These imperfections can beparticularly troublesome in displays with plastic substrates and can beworse for displays of large area. There can also be imperfections in theconductive layer forming the electrodes. The electrode materials are notperfect conductors and being very thin, a small fraction of a micronthick, are subject to non-uniformities in conductivity. The layer ofcholesteric liquid crystal is also subject to variations in thedielectric constant caused by alignment variations in the materialsandwiched between the two electrodes or by variation in the polymericdispersion. Variations in the dielectric constant cause variations inthe electric field across the material when a voltage is applied to theelectrodes.

Uniform reflective colors are important in multiple color displays inwhich different colors can be electronically addressed by stackingcholesteric liquid crystal display cells reflecting different colors.Cells in which the cholesteric materials have pitch lengths reflectingthe primary colors, red, green and blue, are typically stacked. By grayscale addressing each of such stacked display cells, numerous colors areavailable on the display with additive color mixing. Uniform gray scaleof each of the stacked colors is possible with the instant invention forcases where the displays cells have imperfections.

The instant invention reduces the demands on the manufacturing processin that display cells with any of the above imperfections can be used.This invention substantially lowers the display cost in someapplications by enabling use of imperfect display cells but whileachieving high quality gray scale color images.

The phrase “uniform gray scale reflectivity in the display area” or thelike as used in this disclosure means that no discontinuities in thereflectivity of gray scale are observable with the naked eye anywhere inthe display area. Gray scale is defined herein as any level ofreflectance of liquid crystal material that is between a maximum levelof reflectance of the planar texture and a minimum level of reflectanceof the focal conic texture of the material. Each active layer ofbistable cholesteric liquid crystal material has a characteristicpredetermined color determined by the pitch length of the material. Whendisplays include two or more stacked active layers, the display canexhibit the color of either active layer or an additive mixture of thecolor of both active layers along with any back color of the lightabsorbing back layer. Gray scale color used in this disclosure means thecolor of the display or images on the display produced when one or moreof the active layers is at a gray scale level. The terms “bistablecholesteric liquid crystal” mean that once one or more areas of thedisplay have been driven to a desired focal conic texture or planartexture (or gray scale level), the display can reflect the focal conictexture and the planar texture (and any gray scale level) withoutapplication of an electric field.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains black and white drawings only.

FIG. 1: Electrooptic curves for drive pulses applied to a conventionalcholesteric liquid crystal display;

FIG. 2: Typical drive pulses of prior art: (a) DC, (b) AC, (c) AC with apause between phases;

FIG. 3: Drive pulses of prior art with pulse width modulation: (a) DC,(b) AC, (c) AC with pause;

FIG. 4: Schematic illustration of an electronic skin in a side andexploded view (not to scale);

FIG. 5: Photograph of a bistable cholesteric cell illustratingnon-uniform and uniform shades of gray relative to the planar texture;

FIG. 6: Electro-optic response curves produced using narrow reductionpulses of various pulse widths in the present invention yielding uniform(solid data points) and non-uniform (open data points) gray scalereflectivities for an electronic skin made of glass substrates;

FIG. 7: Figure used in Example 4 to demonstrate the cumulative nature ofreduction pulses;

FIG. 8: Figure of Example 5 showing plot of 16 levels of uniformreflectance obtained by reduction pulses;

FIG. 9: Figure of Example 5 showing plot of 8 levels of uniformreflectance obtained by reduction pulses;

FIG. 10: Figure of Example 5 showing plot of 4 levels of uniformreflectance obtained by reduction pulses;

FIG. 11: Figure of Example 6 showing the electrooptic response curve forreduction pulses yielding uniform (solid data points) and non-uniform(open data points) reflectivities for an electronic skin made of plasticsubstrates;

FIG. 12: Schematic illustration of a stacked full-color electronic skinin exploded view (not to scale) wherein Cells A, B and C areblue-reflecting, green-reflecting and red-reflecting, respectively; and

FIG. 13: Color photos of electronic skin on a cell phone housing wherein131, 132 and 133 depict a cell phone including a green, blue or redelectronic skin, respectively.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the present invention features electronic skin thatincludes substrates, electrically conductive layers and a singleoptically active layer of bistable cholesteric liquid crystal materialas illustrated in FIG. 4 which shows a side view 40 a and an explodedview 40 b. The electronic skin of this embodiment has outer substrates,an upper transparent substrate, 41 and a lower substrate, 42 which maybe transparent or opaque. Both substrates each have an electrode layerdisposed on their inside surface. One or both of the substrates may bepatterned or unpatterned. The illustration of FIG. 4 has patternedelectrodes 43 on the upper substrate and unpatterned electrodes 44 onthe lower substrate. An active layer 45 is disposed between the twosubstrates, 41 and 42 and their corresponding electrodes 44 and 43. Theactive layer is a bistable cholesteric liquid crystal preferably in theform of a polymer dispersion 45 as illustrated. The dispersion may be inthe form of cholesteric liquid crystal droplets or the polymer may be anetwork dispersed within the cholesteric material. The cholestericmaterial has a pitch length effective to reflect light of a certaincolor. This color is determined when the display is in the planartexture of maximum reflectance reflecting ambient light (e.g., roomlighting). The device has a light absorbing back layer 46, which can beblack or any color. Incident light not reflected by the active layer iseither totally or partially absorbed by the back layer.

The electronic skin has display areas (regions where the liquid crystalis bounded by the electrodes) that are susceptible to discontinuities ingray scale reflectivity. FIG. 5 shows a photograph 50 of an electronicskin so constructed according to the schematic illustration of FIG. 4.The dimensions, materials and addressing pulses are described inExample 1. Region 51 is the planar texture addressed with an erase pulseof prior art. Region 52 is a region addressed with a gray scale pulse ofprior art and clearly shows the non-uniformity or blotchiness of theresulting image.

Once the reduction pulses have been applied to the electrodes inaccordance with the invention, the active layer reflects light at a grayscale having uniform reflectivity in the display area as shown in region53 of FIG. 5. The electronic skin can be electronically switched todisplay different gray levels and consequently different colors if theback layer is some other color than black. A driver device appliesreduction voltage pulses to electrical leads connected to the electrodesof the active layer effective to enable the active layer to reflectlight at gray scales having uniform reflectivity in the display areas.The electronic skin exhibits an electronically changeable color that isan additive mixing of the color reflected by the active layer at thevarious gray scale levels and any color reflected by the back layer. Forexample, the cholesteric material may reflect green and the back layerred. The resulting color perceived depends on the level of reflectivityof the green layer.

The electronic skin has uniform gray scale reflectivity by applying oneor a series of voltage reduction pulses to the conductive layers,creating an electric field across the active layer. Each reduction pulsepartially reduces the reflectivity of the active layer. The instantinvention reduces the sensitivity of reflectance to voltage by placingthe active layer in a desired gray scale level using short or narrowreduction drive pulses. The pulses are sufficiently short to reduce theelectronic skin's reflectance sensitivity to such cell imperfections ascaused by variations in the cell gap thickness, variations in electrodeconductivity, or variations in the cholesteric or cholesteric dispersionmaterial.

FIG. 6 shows an experimental plot 60 of the response of the reflectivityof the liquid crystal material to short reduction pulses as described inExample 2. Four response curves are shown corresponding to each of fourdifferent narrow pulsewidths. These pulse widths are substantiallysmaller than the 100 ms pulse width of FIG. 1. As described in Example2, each of the points in plot 60 in FIG. 6 were made by first applyingan erase pulse, which erased the active layer to the planar texture. Theerase pulse was then followed by a single pulse of narrow pulse width.Experimental plots using four different pulse widths: 6 ms-167 Hz(circle data points); 1.2 ms-833 Hz (square data points); 0.7 ms-1429 Hz(upright triangle data points) and 0.5 ms-2000 Hz (inverted triangledata points) are shown in plot 60. In each of the four experimentalplots the data points were made by applying an erase pulse, which erasedthe active layer to the planar texture, followed by applying reductionpulses of narrow pulse width indicated above and in FIG. 6.

A desirable feature of these curves is that each curve has portions onthe curve (uniformity regions) where the resultant reflectivity is lesssensitive to the pulse voltage and thus, less sensitive to variations inthe voltage due to display imperfections. The uniformity portions orregions of each curve where the resultant reflectivity is uniform overthe entire skin surface is indicated by solid points on the curvewhereas those portions of the curve that result in non-uniform orblotchy reflectivity are indicated by open points. It is also apparentby observing the minimum values of the reflectance of each curve that itis not possible to transition fully from the planar state to the darkestfocal conic state in a single pulse as compared to a 100 ms pulse, seevoltage V3, 13 of FIG. 1. The curve minimums of FIG. 6 do not achieve aminimum reflectance value of 0.2 arbitrary units (a.u.) that is achievedby curve 11 of FIG. 1 because the widths of the pulse are too narrow.Each of the narrow pulses drive the display partially towards reducedreflectivity (darker gray level); however, they do not achieve thedarkest level of the focal conic state no matter what voltage is used.However, if reduction pulses are later discovered that do achieve thedarkest level of the focal conic state, this should not be used to limitthe invention. It will be shown later that because of the cumulativenature of bistable cholesteric materials multiple pulses can be used toreduce reflectivity even further.

The reduction in reflectivity caused by a pulse can be controlled byadjusting the width and the voltage of the pulses. We now return to thecurve of FIG. 6 that shows the effect on the electrooptic response curveon reflectance using pulses of 6 ms pulse widths at various voltagesfrom initial state of the planar texture obtained by first applying anerase pulse. It is apparent that the reflectivity passes through aminimum but does not achieve the low reflectance of the focal conicstate that was achieved in FIG. 1. After reaching the minimum, thereflectivity rises and achieves the value of the planar texture athigher voltages. One of the inventive features of this disclosure isthat a small region of this curve, indicated by the solid data points61, yields a uniform reflectance over the display or electronic skin(i.e., across the entire display area) whereas the other open datapoints 62 on the curve yield a non-uniform reflectivity. Turning now tothe curve of FIG. 6 obtained with 1.2 ms pulses it is apparent that thatcurve also passes through a minimum; however, the minimum value ofreflectivity achieved is not as low as that of the 6 ms pulse. It is tobe further noted that the region of the curve indicated by solid datapoints 63 covers a larger voltage range than that of the 6 ms pulse.Likewise the solid data points of the 0.7 ms width pulses 64 cover alarger voltage range but do not reduce the reflectivity as much as the 6ms or 1.2 ms width pulses. The solid data points of the 0.5 ms widthpulses 65 cover the largest voltage range of all pulse widths indicatedbut provide the least reduction in the reflectivity,

It is to be further noted that the solid data points 61, 63, 64, and 65for the 6 ms, 1.2 ms, 0.7 ms and the 0.5 ms pulse widths, respectively,all are on regions of the curve with a small slope. These are regions ofthe curves where the reflectance is least sensitive to changes in thevoltage. The reduction pulses associated with regions 61, 63, 64 and 65are insensitive to the voltage level differences provided byimperfections in the electronic skin from which they were achieved. Itis shown in Example 3 that one can determine the magnitude of theimperfections from these regions of the curve. Example 3 uses the soliddata point region 61 for the 6 ms pulse width to measure the variationin cell gap of the electronic skin from which the data were obtained. Avalue of 0.35 μm for the maximum cell gap variation compares favorablywith a value of ˜0.4 μm expected from estimated maximum variation of thesurface of the unpolished glass used to make the electronic skin.

For the purposes of this invention a reduction pulse is defined as apulse that achieves gray scale in which the pulse width is such that asingle reduction pulse does not reduce the reflectance to the minimumreflectance of the focal conic state of the display for any pulsevoltage. For example, the minimum of the reflectance of FIG. 1 is 0.2a.u. In most cases, such as that of Example 2, it is not necessary toeven be very close to that minimum value because more than one and oftenseveral pulses are used, as will be demonstrated below, to achieve adesired gray level. Not all reduction pulses produce uniform graylevels. The range of pulse voltages and widths that do provide uniformgray levels depend upon the imperfections in the cell such as the degreeof variation in cell gap spacing of the electronic skin device. As notedin the discussion of FIG. 6 of Example 2, the regions of uniform graylevel (solid points) have the following properties:

1) Occur at slopes on the voltage response curve where the curveapproaches its minimum value. This is believed to be because the minimumvalue is the portion of the curve least sensitive to imperfections inthe electronic skin.

2) All terminate on the left side of the voltage response curves atabout the same slope for curves obtained with different reduction pulsewidths. The point of termination is where the pulse widths becomesensitive to the skin imperfections. The termination points will bedifferent for electronic skins with different imperfections and are bestdetermined by experiment which can be easily carried out by one ofordinary skill in the art in view of this disclosure without undueburden.

3) Extend over a larger voltage scale at smaller reduction pulse widths.Smaller pulse widths sample a larger range of small slopes on thevoltage reductions curve. For this reason, small pulse widths are oftendesired in obtaining uniform gray levels.

4) Provide smaller reductions in gray level at smaller reduction pulsewidths. As will be shown immediately below, small pulse reductions arenot an inconvenience as a series or sequence of small pulses arecumulative in their reduction effect.

One or more pulses in succession at the same voltage can be used toachieve each gray level step reduction. This is because of thecumulative nature of bistable cholesteric liquid crystal displays (seeU.S. Pat. No. 6,133,895). The cholesteric material possesses a memorysuch that a gray level following a pulse is not only retained by thepulse but that a second pulse adds to the memory to advance the graylevel in a cumulative fashion. FIG. 7 illustrates the use of five acreduction pulses 72 following an erase pulse 71 to drive to a midlevelgray. The reflectance 74 from the erase pulse 71 and the reflectance 75from the reduction pulses 72 are shown on the left side of the FIG. 7graph whereas the voltages for the erase and reduction pulses are shownon the right. Note that the planar erase sequence resets the display toa bright state from any gray level state.

Reduction in brightness does not necessarily require a planar erasesequence; they can be achieved by a sequence of reduction pulses whichreduce the brightness to the desired gray level based on the initiallevel. Here, the drive waveforms depend on the relationship between theinitial gray level and final gray level state.

The invention can produce one desired gray level reduction for eachpulse. If pulses of fixed amplitude are used, the pulse width of eachsuccessive pulse may be varied if desired so as to achieve the desiredlevel of gray for that pulse. Example 5 demonstrates the use of 15reduction pulses to create 16 uniform reflectance levels (levels atpulses numbered 1 through 15 being gray scale levels) where each levelof gray corresponds to a particular pulse number. An experimentallyderived plot of gray scale between the planar and focal conic texturesof Example 5 is shown in FIG. 8. The level of reflectance of each grayscale level is arbitrarily chosen. One would specify a desired number ofreflectance levels from the maximum reflectance (1.0 a.u.) toward thelowest reflectance (above 0.2 a.u.), which here was 16 reflectancelevels in the sequential series. Then each gray scale (at reductionpulses numbered 1-15) has about 1/16 of the overall reflectivity changeof the series. That is, each gray level can be equally spaced withregard to the change in reflectance achieved from the previousreflectance level. The reflectance of a cell, curve 81, for 16reflectance levels driven first by a planar erase pulse, followed by apause, and then from zero to fifteen reduction pulses are appliedwherein each pulse begins 16.7 ms (60 Hz frequency) after the previouspulse begins. Each pulse decreases the reflectance by one gray level inthis figure, so that a desired gray level may be created by driving onlythe corresponding number of reduction pulses. In FIG. 8 for example, a“5^(th) gray level”, 82, yields a reflectance value of 0.73 a.u. asachieved by applying 5 pulses, following the erase pulse thatinitializes the active layer in the planar state. In Example 5, thereduction pulses all have the same voltage; however, the widths areadjusted to create equal gray scale reductions. The pulse widths beganshort and trended towards longer values as reflectance decreased. Thepulse voltage was determined such that pulse widths yielded a uniformgray level. It was found however that it was not critical if the lastone or two pulses be in the range of uniform gray as indicated by FIG. 6since a sequence of reduction pulses (more than one reduction pulse)tends to extend the voltage range of these curves; that is, the datapoints of FIG. 6 were made using only one reduction pulse and more thanone reduction pulse tends to extend the range of solid data points.Example 5 further shows that eight levels of uniform reflectance (sevenreduction pulses) (FIG. 9) as well as four levels of uniform reflectance(three reduction pulses) (FIG. 10) were also possible in the particularbistable cholesteric cell used in the experiment.

The voltage of a pulse may also be varied to tune the reduction inbrightness caused by the pulse. However, relatively higher voltages arerequired to increase brightness if the pulse widths are too short. Lowerplanar erase voltages may be used with longer pulses in a manner knownin the art to reset the display to the maximally bright planar texture.In the preferred embodiment of the instant invention, the display isdriven first with a planar erase sequence, a pause of variable length topermit the LC to relax and then a sequence of reduction pulses whichreduce the brightness to a desired gray level. A pulse frequency of 60Hz is desirable to eliminate flicker and also to eliminate beatfrequencies visible when the display is viewed under ac room lightsources. The reduction pulse scheme can also be applied to a passivelydriven matrix consisting of row electrodes on one substrate and theorthogonal column electrodes on the other substrate.

The greatest flexibility in driving comes if both voltage and reductionpulse width may be tuned. Consider the reduction pulse curves in FIG. 6.If voltage may be varied, a single 0.5 ms reduction pulse can produceuniform gray levels in the range of ˜0.9 a.u. to ˜0.64 a.u. Likewise, asingle 0.7 ms pulse can produce uniform grays over the range ˜0.78 a.u.to ˜0.6 a.u., and a single 1.2 ms reduction pulse can produce uniformgrays over the range ˜0.6 a.u. to ˜0.48 a.u.

Thus, any given reflectance between ˜0.9 a.u. and ˜0.48 a.u. may bedriven from planar with a single reduction pulse by selecting theappropriate pulse width to operate on one of these three curves andadjusting voltage accordingly. The inclusion of even more curves (pulsewidths) should permit uniform gray levels over the full reflectivityrange, where each change in gray level is achieved by applying a singlereduction pulse (or more than one reduction pulse) of appropriatevoltage and pulse width. For example, one can draw a “uniformity line”that intersects all of the uniformity regions (dark datapoints) of thepulse width curves of FIG. 6; choose a series of gray scale levels; andidentify intersection points where each of the gray scale levelsintersects the uniformity line, wherein the gray scale reduction pulsescomprise pulse widths and voltages on or near each of the intersectionpoints. That is, one would carry out the step of selecting voltages andpulse widths which, based on the uniformity regions of the curves, willproduce the gray scale reduction pulses that produce the gray scalelevels in the series.

Electronic skins of principal interest are those that are made offlexible plastic substrates. It is of interest to examine the use ofreduction pulses for achieving uniform gray levels in such skins wherethe uniformity might be expected to be a larger factor than for glass.FIG. 11 shows the electrooptic response curve for an electronic skinwith flexible plastic substrates. The curve was obtained experimentallyfor a flexible plastic electronic skin as described in Example 6. Eachpoint on the curve shows the reflectance versus the voltage of a singlereduction pulse at a constant width of 2 ms. Each reduction pulse waspreceded by a planar erase pulse. The solid points on the curve indicatethose pulses that yield a uniform gray level. The open data points arethose that yield a non-uniform gray level. The solid data points on thecurve were analyzed similar to that of Example 3 to determine thevariation in cell gap thickness for the electronic skin made of flexibleplastic substrates. The analysis indicated a variation of ˜0.6 μmslightly larger than that obtained for the unpolished glass substratesof Example 3; however not out of range of the experimental error ofbeing the same.

In a second embodiment shown in FIG. 12 the electronic skin includes twoor three layers of cholesteric liquid crystal stacked over each other.The display can include one or more substrates. One of these substratescan form an outer surface of the electronic skin and one or moreinternal substrates can be located between adjacent active layers. Theother outer surface of the electronic skin can be a transparent polymersubstrate or a protective layer coated over the upper electrode (see forexample U.S. Pat. No. 7,170,481) then applied to an article; the otherouter side of the electronic skin might be attached to a surface of thearticle with or without an intervening substrate.

A preferred embodiment of the electronic skin is illustrated in FIG. 12consisting of stacked cells, A, B, and C in which cells A and B sharethe same substrate 121 and cell B and C share the same substrate 122.The electronic skin might include two substrates between adjacent activelayers (in the case of both the double and triple stack displays) witheach substrate having an outer electrode just for one active layer (anindex matching material being disposed between and matching the index ofrefraction of the two substrates). The electronic skin may also be onein which all of the layers including the electrodes, active cholestericliquid crystal dispersions and substrates are coated as for example inU.S. Pat. No. 7,170,481. The electronic skin shown includes threestacked active layers with electrically conductive layers 125, 126 and127 flanking (disposed on either side of) each of the active layers. Theelectronic skin includes upper 120 and lower substrates 124. Upper andlower cells A and C each include two substrates, a liquid crystal layerand opposing electrodes on the sides of it; and intermediate cell Bincludes a substrate from each of the upper and lower cells, a liquidcrystal layer and opposing electrodes on the sides of it. A lightabsorbing back layer 128 is disposed at the back of the electronic skin.The back layer is black or any color as described above. Preferredreflective colors of the three cells are red, green and blue in anyorder. Electrical leads are connected to each electrode layer. Theelectrodes can be in the form of patterned row and column electrodesdefining separate display segments, the electrodes of the display can beunpatterned, or both.

The electrodes used on the substrates can be solution coated conductingpolymer, conducting carbon, silver, carbon nanotubes, or the electrodescan consist of vapor deposited materials (e.g., indium tin oxide, indiumtin zinc oxide), combinations of these materials or other electricallyconductive materials. The substrates can be formed of rigid or flexibleglass or polymer (e.g., PET, PC, cyclic olefins, or PEN polymers). Theback coating can consist of black or any color paint or absorbing dyethat is applied in a number of techniques such as spraying, screenprinting, ink jet printing, or other application technique.

The electronic skin can be flat, curved, or conformed to athree-dimensional curved surface. The electronic skin can be simply bentcurved by flexing the film or it can be thermoformed (or created byanother means) to an arbitrary shape. The electronic skin can bethermoformed with standard thermoforming techniques whereby athermoforming mold is built of the desired shape, the mold is heated,and then the electronic skin is placed in the mold under temperature.After the mold is cooled the electronic skin is removed. The temperaturerequired for thermoforming is the glass transition temperature of thesubstrates of the electronic skin. Electronic skins can be thermoformedto curvatures that would otherwise cause the ChLC layer to delaminate.

Electronic skin can include different color cholesteric liquid crystal,ChLC, layers, such as different dispersions of ChLC in a polymer matrixformed by a polymerization induced phase separation, PIPS, process (asdisclosed in U.S. Pat. No. 7,351,506), thermally induced phaseseparation, TIPS, process or solvent induced phase separation, SIPS,process (as disclosed in U.S. Pat. No. 4,685,771).

FIG. 13 shows a color photograph of cell phone housing with anelectronic skin of the present invention. The electronic skin was madewith a triple stack consisting of the primary colors red, green andblue. The cholesteric material used in the prototype is of a PIPS typeof U.S. Pat. No. 7,351,506. Between each pair of adjacent liquid crystallayers of the triple stack display was an internal substrate of flexiblePET polymer less than 1 mil in thickness having shared electrodes. Thered, green and blue colors for the phone were achieved by addressing theelectronic skin with drive electronics separate from the phone. However,one of ordinary skill in the art can apply appropriate erase andreduction voltage pulses to the electronic skin using the cell phoneelectronics and power. Using uniform gray levels the color of theelectronic skin can be electronically tuned to a desired color.

Full-color, single substrate flexible displays that can be modified toform the inventive electronic skin are described U.S. Pat. No. 7,170,481using emulsion materials or by using polymerization induced phaseseparation (PIPS) materials. Electronic skins can also be made onsubstrates that can be prepared and released from a release layer (U.S.Pat. Nos. 7,796,103, 7,773,064, and 7,236,151). Electronic skins may beprepared by emulsions on a transparent substrate such that afterlamination the skin is viewed through the substrate similar to processesdescribed in U.S. Pat. Nos. 6,359,673 and 6,788,362. Electronic skinsmay also be prepared by coating the emulsion or phase separatedmaterials directly on the device itself where the device surface is thesubstrate upon which other coatings such as the electrodes are alsocoated.

Electronic articles or devices that can include or be composed ofelectronic skin of the invention and can be changed to any desiredcolor, are described in the following, non-limiting list: cell phone,laptop, computer, computer monitor, computer mouse, computer keyboard,television, I-pod, MP3 player, PDA, video game controller, stereo,radio, CD player, appliance, toy, headphones, clock, handheld electronicdevices and combinations thereof. Further such articles include: keyring accessory, shoe, purse, backpack, briefcase, computer case,computer covering, jewelry, watch, bottle, bottle lid, clothing,clothing embellishment, furniture, furniture embellishment, mobileentertainment case.

The article can include a support surface on which the electronic skinis attached or the entire article or portion thereof can be composed ofthe electronic skin. The support surface can have any shape. The supportsurface can include a 3 dimensional contour in which case any outersupport layers and substrates of the electronic skin are formed of apolymer material enabling the electronic skin to be conformable to thecontour of the support surface. The electronic skin can be drapable, asin a drapable sheath or covering, or a fabric, textile, paper orpolymeric sheet composing the electronic skin, the components of theelectronic skin being constructed in a manner disclosed in U.S. Pat. No.7,170,481 as would be apparent to one of ordinary skill in the art inview of this disclosure. Examples of articles composed partly orentirely of the electronic skin are drapable sheaths such as laptopcovers and fabric made of the electronic skin as where the fabric is asubstrate of the skin.

The invention will now be described by reference to the followingexample presented to illustrate, rather than limit, the invention.

Example 1

Levels of gray are levels of brightness as they are reduced from themaximum brightness exhibited by the planar texture of a particularelectronic skin. In order to demonstrate the difference between uniformand non-uniform levels of gray, a bistable cholesteric cell illustratedin FIG. 4 was constructed. The glass cell was constructed using two5.5×5.5 inch glass substrates, each 0.55 mm thick with ITO transparentconductors. The glass substrates were of unpolished glass exhibiting anexpected roughness and not a perfectly smooth surface. The cell gapspacing was set by the use of glass spacers with an average size of 9microns. However, because of the roughness of the substrates the cellgap was expected to have imperfections in cell gap thickness by as muchas 0.4 μm which is typical for the use of unpolished glass. The glasscell was filled with a cholesteric liquid crystal KLC-23, a KentDisplays, Inc. proprietary mixture of a nematic liquid crystal with alarge dielectric anisotropy to yield a low voltage response and chiraladditives at a sufficient concentration to yield a Bragg reflection ofapproximately 530 nm. (This example is not sensitive to the type ofmaterial used. Non-proprietary nematic mixtures such a E-44 combinedwith BL061 (a cholesteric mixture composed unknown chiral additives andE44), both obtained from EMD Chemicals, tuned to same Bragg reflectivewavelength could be used as a suitable material; however, the operatingpulse voltages and widths would be different to achieve the sameresults.) The conducting electrodes of the cell were connected to abipolar power supply (by LC Technologies, Inc.) that outputted awaveform of voltage pulses specified by a polynomial waveformsynthesizer (Model 2020 by ANALOGIC) that was programmed through thecomputer program, LabVIEW.

Cell 50 of FIG. 5 shows the display with the top half erased to theplanar texture (area 51) with a 250 Hz AC erase pulse of amplitude of 50V and pulse width of 100 ms. Area 52 is first erased in the same mannerwith a 50V, 250 Hz, 100 ms wide pulse then followed 300 ms later with a41.5 V, 250 Hz AC pulse of 100 ms in width driving the pixel to anintermediate gray level. The non-uniformity or blotchy appearance of thegray level in area 52 is clearly evident. Area 53 is driven to anintermediate gray level with reduction pulses of the instant invention.Area 53 is first erased to the planar texture with a 250 Hz AC erasepulse of an amplitude of 50 V and pulse width of 100 ms then 300 mslater driven to the intermediate gray level 53 with a sequence ofreduction pulses. The reduction pulse sequence consisted of thirteen45V, one cycle bipolar pulses of varying pulse widths with a period orspacing of 16.7 ms. The widths of the reduction pulses (in ms) toachieve the gray level shown were: 0.42, 0.35, 0.36, 0.40, 0.40, 0.40,0.45, 045, 0.50, 0.50, 0.55, 0.57, 0.75, respectively. The gray level ofarea 53 is clearly uniform, and completely void of the blemishes shownin area 52.

Example 2

Electrooptic response curves which are a plot of the reflectance of thecell versus the rms voltage of a pulse are experimentally achieved for abistable cholesteric cell for various pulse widths as shown in FIG. 6.While acquiring each of the data points, the cell was observed visuallywith the eye to determine if the cell exhibited a uniform or non-uniformgray level. The glass cell was constructed using two 2×2 inch glasssubstrates, each 1.0 mm thick with ITO transparent conductors. The glasssubstrates were of unpolished glass exhibiting an expected roughness andnot a perfectly smooth surface. The cell gap spacing was set by the useof glass spacers with an average size of 5.0 microns. However, becauseof the roughness of the substrates the cell gap was expected to haveimperfections in cell gap thickness by as much as 0.4 μm expectedforunpolished glass. The glass cell was filled with the same cholestericliquid crystal KLC-23, (Kent Displays, Inc.) as used in Example 1.Conductive leads were connected to the upper and lower ITO electrodes ofthe cell which were then connected to a bipolar power supply (by LCTechnologies, Inc.) driven by a polynomial waveform synthesizer (Model2020 by ANALOGIC) to provide voltage pulses to the cell. The opticalresponse was measured by use of a 300 W Xenon Arc Lamp (Model 6258 byORIEL Instruments) that emits a white light into a grating monochromator(Model 77250 by ORIEL Instruments). The monochromator divides the whitelight into individual wavelengths, and transmits the wavelengthspecified. The light is then incident (normal) to the cell, and thelight that is reflected at 45° is measured by a photodetector (Model#PDAMP-4 by LC Technologies, Inc.) The electro-optical response isdetermined by measuring the 45° reflectance of the display afterapplying a pulse sequence consisting of an erase pulse followed after apause by a reduction pulse. The reflectance is the measured lightintensity normalized to a value of one at its maximum recorded valuefrom the planar texture. With this definition of reflectance the unitsare given as arbitrary units (a.u). The reflectance was measured afterwaiting several seconds to allow time for the liquid crystal to relax toits equilibrium state. Before applying the next pulse sequence to obtainthe next data point the cell was visually inspected to determine whetherthe gray level was uniform or non-uniform.

Each data point on the response curves of FIG. 6 was obtained with aplanar erase pulse made up of a 50V ac square wave of 250 Hz pulse of200 ms in width then applying a narrow reduction pulse after a pause of16.7 ms to drive the cell into a gray level. The pulse widths of thereduction pulses were chosen substantially smaller that the erase pulsewidth (200 ms erasing pulse for the combined plurality of pulses havingthe width of the pulse 71 on the x axis in FIG. 7) such that a singlereduction pulse never achieved the minimum reflectance of the focalconic state. The reduction pulses used in obtaining FIG. 6 were eachsingle square bipolar pulse of pulse widths: 6 ms (circle data points);1.2 ms (square data points); 0.7 ms (upright triangle data points) and0.5 ms (inverted triangle data points). The open data points in FIG. 6are those data points where the gray level of the cell was observed tobe non-uniform whereas the solid data points are those where the cellwas observed to be uniform. As noted in the text, the regions of uniformgray level (solid points): 1) occur where the slope of the curve issmall; 2) all terminate on the left side of the curves at about the sameslope; 3) extend over a larger range of voltages for smaller reductionpulse widths; 4) provide smaller reductions in gray level at smallerreduction pulse widths.

Example 3

In this example, while not wanting to be bound by theory, we estimatedthe variation of the cell gap distance from the data of FIG. 6 based onthe assumption it is due to the variation in the surface roughness ofthe unpolished glass surface of the substrates from which the cell wasformed. The premise of this example is that the slopes of the curves inFIG. 6 determine the sensitivity of gray levels to the imperfections inthe bistable cholesteric cell. This is supported by the observation thatthe solid data points (points exhibiting uniform gray levels) allterminate on the left side of each curve at about the same slope. Thisvalue is given by the ratio (ΔR/ΔV)_(max) where ΔR is the change inReflectance and ΔV is the corresponding change in Voltage at a point onthe curve. From the curves it is found that (ΔR/ΔV)_(max)≈0.02. If thenon-uniformity is due to variations in cell gap then one obtains:(ΔR/ΔV)_(max)=(ΔR)/E(Δd)≈0.02.  Eq. 1

Where E is the electric field and Δd is the variation in cell gapspacing d. Solving:(Δd)_(max)≈(ΔR)_(max) /E(0.02)  Eq. 2

Eq. 2 states that for a cell gap variation (Δd)_(max), there is acorresponding variation in (ΔR)_(max) that is insensitive to thisvariation. One can get that variation in (ΔR)_(max) from one of theelectrooptic response curves of FIG. 6 and seeing where on this curvethe uniform gray occurs. For this purpose, we choose the plot for the 6ms pulse width (reason to be discussed later). We see that uniform grayoccurs at a field nearly symmetric around its minimum value of aboutE≈22V/5 μm=4.3 V/μm. We further see that at this field (ΔR)_(max)≈0.03.This is then deemed to be the variation in (ΔR)_(max) that isinsensitive to (Δd)_(max) for which one obtains: (Δd)_(max)≈0.03/4.3(0.02)≈0.35 μm. This appears comparable with the estimated roughness ofthe unpolished glass cell, ˜0.2 μm, in which case the cell gap variationwould be ˜0.4 μm.

The plot with the largest pulse width, 6 ms was chosen for thiscalculation because the effect of the response times of the liquidcrystal was ignored in the calculations. It may be reasonable to expectthat the shorter pulse widths are not as accurate since the liquidcrystalline material does not have time to respond fully to the pulse.It is expected that the curves should be symmetric about the minimum andthey are not and get more asymmetric as the pulse width gets narrowerwhich is perhaps another result of response of the liquid crystal beingtoo slow to fully respond to the applied pulse as well as thecholesteric material being partially driven into the homeotropic(untwisted nematic structure) texture at the higher voltages.

Example 4

In order to demonstrate the use of cumulative pulses a cell wasconstructed identical to that of Example 2 except that the cell wasfilled with a cholesteric liquid crystal KLC-22 a Kent Displays, Inc.mixture with a Bragg reflection of approximately 530 nm. As indicatedearlier, this example is not sensitive to the type of material used andcommercial materials from EMD Chemicals such as nematic mixtures such asE-44 with chiral additive BL061 can be obtained with the same results;however, the operating pulse voltages and widths would be different toachieve the same results. The electrodes of the sample were connected tothe same voltage source as in Example 2 and in addition connected to anoscilloscope to observe the pulse shapes. The reflectance was measuredas in Example 2. The results shown in FIG. 7 are for a pulse sequenceconsisting of a 250 Hz, 200 ms, 50.0 volt erase pulse followed after apause of 450 ms with a series of 45 volt single square wave reductionpulses of: 0.42 ms, 0.35 ms, 0.36 ms, 0.40 ms, and 0.40 ms pulse widths,respectively, with a pause in between each reduction pulse such that anew reduction pulse begins every 16.7 ms (60 Hz application rate). Thecumulative nature of the reduction pulses is evident in the FIG. 7.

Example 5

The use of reduction pulses to achieve a sequence of uniform gray levelsdepending upon the number of pulses is demonstrated by using the samecell as Example 2. FIG. 6 and Example 2 were used to determine the rangeof pulse voltages and widths to use for reduction pulses that provideuniform gray levels. It is convenient to choose a fixed voltage and fromFIG. 6 to determine the range of pulse widths that can provide uniformgray. In this Example we choose 45V in which case the pulse widths thatcan provide uniform gray are any width shorter than approximately 1.0ms; however, if the pulses are shorter than 0.5 ms the amount of grayscale reduction will be smaller. FIG. 8 shows the reflectance for 16levels of uniform reflectance with approximately equally spaced changesin gray level versus reduction pulse number. A 50V, 200 ms, 250 Hz pulsewas used to erase the cell to the planar texture. The various shades ofuniform gray were then obtained by applying a series of reduction pulseseach consisting of one bipolar square wave with a pulse width (onecomplete cycle) depending upon that needed to obtain 16 equal gray levelshifts (i.e., approximately the same reduction in reflectance isachieved by each gray scale level). The pulse widths for each of thereduction pulses that were used were: 0.44, 0.43, 0.42, 0.43, 0.45,0.44, 0.47, 0.47, 0.51, 0.52, 0.60, 0.62, 0.76, 0.86, and 1.10 ms,respectively. A pause was placed between each reduction pulse such thatnew pulses begin every 16.7 ms (60 Hz application rate) in order toprevent aliasing with the 60 Hz room light incident on the cell whilethe pulses were being applied. From FIG. 8 one can choose a uniform graylevel based upon the number of pulses to be applied. For example, toproduce the first reflectance level (reflectance of 1.0 a.u.) 0reduction pulses are applied whereas to achieve the gray scale levelnumber 4, reduction pulses 1 to 3 are applied in succession.

FIG. 9 shows a similar example but with 8 reflectance levels. This plotwas achieved in the same way as the 16 level scheme above except thatfor producing the gray scale series the reflectance produced by the cellwas divided into 8 segments (seven reduction pulses and gray scalelevels). A 50V, 200 ms, 250 Hz pulse was used to erase the cell to theplanar texture. The eight reflectance levels were then obtained byapplying a series of reduction pulses each consisting of one bipolarsquare wave with a pulse width (one complete cycle) depending upon thatneeded to obtain the equal gray level shifts. These are found to be:0.54, 0.49, 0.52, 0.58, 0.62, 0.80, 1.10 ms respectively. A pause wasplaced between each reduction pulse as before such that new pulses beginevery 16.7 ms (60 Hz application rate) to prevent aliasing with roomlights while the pulses were being applied. From FIG. 9 one can choose auniform gray level based upon the number of pulses to be applied. Forexample, to produce the first reflectance level (reflectance of 1.0a.u.) 0 reduction pulses are applied whereas to achieve the gray scalelevel number 4, reduction pulses 1 to 3 are applied in succession.

FIG. 10 shows a similar example but with 4 reflectance levels (3reduction pulses and gray scale levels). This plot was achieved in thesame way as the 16 level and 8 level schemes above except that thereflectance for the cell was divided into 4 levels. A 50V, 200 ms, 250Hz pulse was used to erase the cell to the planar texture. The shades ofuniform gray were then obtained by applying a series of reduction pulseseach consisting of one bipolar square wave with a pulse width (onecomplete cycle) depending upon that needed to obtain equal gray levelshifts. These are found to be: 0.70, 0.74, 1.10 ms respectively. A pausewas placed between each reduction pulse such that new pulses begin every16.7 ms (60 Hz application rate). From FIG. 10 one can choose a uniformgray level based upon the number of pulses to be applied. For example,to produce the first reflectance level (reflectance of 1.0 a.u.) 0reduction pulses are applied whereas to achieve the gray scale levelnumber 4, reduction pulses 1 to 3 are applied in succession.

Example 6

The use of reduction pulses for achieving uniform gray levels in anelectronic skin made of flexible plastic substrates is explored in thisexample. The skin was fabricated from two substrates of 5 mil PETpolymeric films with each substrate being 2×3 inches in size, about thesize of a cell phone housing. The inner side of each substrate wascoated with a conducting polymer to serve as the electrode. The spacingbetween the substrates was controlled by 4.0 μm spherical plasticspacers yielding a cell gap of that thickness. The bistable cholestericmaterial in the cell was a droplet dispersion prepared by the polymerinduced phase separation (PIPS) approach according to U.S. Pat. No.7,351,506. An electrooptic response curve showing the reflectance versusthe voltage of a reduction pulse of width 2 ms was made using the sameapparatus and experimental method of Example 2. The results are shown inFIG. 11 and discussed in the text.

What is claimed:
 1. A method for providing an active layer of bistablecholesteric liquid crystal material with uniform gray scalereflectivity, comprising: plotting electrooptic response curves for saidactive layer showing response of reflectance to reduction pulse voltage,each said curve corresponding to a different reduction pulse width;identifying uniformity regions on said curves at which said active layerhas a uniform gray scale reflectivity; selecting a voltage thatintersects said identified uniformity regions on said curves; choosing aseries of gray scale levels for said active layer that can achieve asubstantially linear decrease in reflectance in response to increasingreduction pulse number; and selecting gray scale reduction pulse widthsat said selected voltage which, based on said pulse widths of saidcurves that intersect said selected voltage, will produce gray scalereduction pulses that achieve each said gray scale level in the seriesat uniform reflectivity.
 2. The method of claim 1 comprising providing adisplay including said active layer disposed between adjacentelectrically conductive layers and a light absorbing layer adapted toabsorb light passing through said active layer, said light absorbinglayer being black or a back color, wherein said gray scale levels of theseries are in a sequential decreasing order, comprising driving saiddisplay to a selected one of said gray scale levels by applying to saidconductive layer all of said reduction pulses in order from a firstreduction pulse, which reduces reflectivity from a preceding maximumreflectivity level, to said gray scale reduction pulse for said selectedgray scale level.
 3. The method of claim 2 wherein said gray scalelevels are spaced such that a decrease in reflectance of each said grayscale level from an immediately preceding said gray scale level issimilar for all said gray scale levels of the series.
 4. The method ofclaim 2 wherein said reduction pulses are unipolar or bipolar.
 5. Themethod of claim 2 wherein said reduction pulses are an AC squarewaveform.
 6. The method of claim 2 comprising changing said gray scalelevel of said display to a gray scale level of higher reflectivity by:applying erasing voltage pulses effective to place said liquid crystalmaterial in said planar texture at said maximum reflectivity; andconducting said step of driving said display to said selected gray scalelevel, where said selected gray scale level is said gray scale level ofhigher reflectivity.
 7. The method of claim 2 wherein said displayincludes at least two stacked said active layers of bistable cholestericliquid crystal material, each said active layer being disposed betweenadjacent said conductive layers, said cholesteric liquid crystalmaterial of said active layers having pitch lengths effective to reflectlight of different predetermined colors than each other, said layer oflight absorbing material adapted to absorb light passing through saidactive layers, said light absorbing layer being black or a back color.8. The method of claim 7 comprising two of said active layers,comprising providing said display with gray scale colors at said uniformgray scale reflectivity by applying said gray scale reduction pulses tosaid electrically conductive layers for one or both of said activelayers.
 9. The method of claim 7 comprising three of said active layers,comprising providing said display with gray scale colors at said uniformgray scale reflectivity by applying said gray scale reduction pulses tosaid electrically conductive layers for one, two or all three of saidactive layers.
 10. A method for providing an active layer of bistablecholesteric liquid crystal material with uniform gray scalereflectivity, comprising: plotting electrooptic response curves for saidactive layer showing response of reflectance to reduction pulse voltage,each said curve corresponding to a different reduction pulse width;identifying a uniformity region on each of said curves at which saidactive layer has a uniform gray scale reflectivity; choosing a series ofgray scale levels for said active layer; and selecting voltages andpulse widths which, based on said identified uniformity regions of saidcurves, will produce gray scale reduction pulses that produce said grayscale levels in the series.
 11. The method of claim 10 wherein said grayscale levels are chosen to achieve a substantially linear decrease inreflectance in response to increasing reduction pulse number.
 12. Themethod of claim 10 comprising identifying a uniformity line thatintersects all of said curves at said uniformity region of each saidcurve, identifying intersection points where each of said gray scalelevels intersects said uniformity line, wherein said gray scalereduction pulses comprise pulse widths and voltages on or near each ofsaid intersection points.
 13. The method of claim 12 comprisingproviding a display including said active layer disposed betweenadjacent electrically conductive layers and a light absorbing layeradapted to absorb light passing through said active layer, said lightabsorbing layer being black or a back color, wherein said gray scalelevels of the series are in a sequential decreasing order, comprisingdriving said display with uniform gray scale reflectivity to a selectedone of said gray scale levels by applying to said conductive layers allof said reduction pulses in order from a first said reduction pulse,which reduces reflectivity from a preceding maximum reflectivity level,to said gray scale reduction pulse for said selected gray scale level.14. The method of claim 13 wherein each said gray scale level isproduced by applying one or more of said reduction pulses.
 15. Themethod of claim 13 wherein said display includes at least two stackedsaid active layers of bistable cholesteric liquid crystal material, eachsaid active layer being disposed between adjacent said conductivelayers, said cholesteric liquid crystal material of said active layershaving pitch lengths effective to reflect light of differentpredetermined colors than each other, said layer of light absorbingmaterial adapted to absorb light passing through said active layers. 16.The method of claim 15 comprising two of said active layers, comprisingproviding said display with gray scale colors at said uniform gray scalereflectivity by applying said gray scale reduction pulses to saidelectrically conductive layers for one or both of said active layers.17. The method of claim 15 comprising three of said active layers,comprising providing said display with gray scale colors at said uniformgray scale reflectivity by applying said gray scale reduction pulses tosaid electrically conductive layers for one, two or all three of saidactive layers.